1. Field of the Invention
The present invention relates to a gradient magnetic field power supply which supplies a current to a gradient coil for producing a gradient magnetic field.
2. Description of the Related Art
With very high speed imaging methods, such as echo planar imaging (EPI), the acquisition of data needed to reconstruct one magnetic resonance image is finished in some tens of milliseconds. In order to implement vary high speed imaging, it is required to cause a gradient magnetic field to rise rapidly. A general gradient coil has an inductance of some millihenries and a resistance on the order of one ohm to hundreds of milliohms. In the ordinary imaging method, a current of 100 to 200 amperes is used to generate a gradient magnetic field of about 1 G/cm. To provide a current of 150 amperes and a coil inductance of one millihenry within one millisecond, a supply voltage of 150 volts are required, assuming that there is no resistance component involved. In the very high-speed imaging method, it is necessary to generate a current about three times greater, that is a high intensity gradient magnetic field, within about five to ten times shorter time. For example, to generate a current of 450 amperes within 200 microseconds, a voltage of 2250 volts must be applied. Gradient magnetic field power supplies generally used with magnetic resonance imaging apparatus have a capability of providing an output voltage of at most 200 volts, which does not meet the above condition. Thus, various measures have been taken so far to accomplish the fast rise of a gradient field required with the very high speed imaging through the use of general gradient magnetic field power supplies.
A first measure is to employ resonance for the purpose of raising a current supplied to a gradient coil. Between a power supply and a gradient coil is connected a capacitor, which constitutes a resonant circuit with the coil. Owing to resonance, a current rises rapidly. When the current reaches a predetermined level, the capacitor is removed, so that the coil is directly connected to the power supply. The power supply then supplies a current of constant magnitude to the coil.
Note that the power supply comprises a negative feedback amplifier the feedback circuit of which consists of a resistor and a phase compensation capacitor.
When a resonance phenomenon is used to raise a current as described above, the load impedance of the power supply varies with time. During the interval when the current is rising, the load impedance is determined by the capacitance C of the capacitor and the inductance L and the resistance R of the gradient coil. When the current is constant, the load impedance is determined by the inductance L and the resistance R of the gradient coil.
With variations in the load impedance with time, the feedback circuit will not achieve proper phase compensation. Therefore, the use of resonance serves to raise the current fast on the one hand, and lowers the stability of the power supply on the other hand.
In addition, when the resonance is employed so as to cause the current to rise fast, the current rise time will depend on the time constant of the resonant circuit. It is, therefore, impossible to regulate the rise time.
A second measure is to support the main gradient field power supply with an auxiliary high-voltage power supply when the current to the gradient coil rises.
FIG. 1 shows an arrangement of such a gradient magnetic power supply as uses an auxiliary power supply. Switches SW1 to SW4 are connected in a bridge configuration. A series combination of a main power supply and a gradient coil is connected between two points, each on a respective one of the two branches of the bridge. An auxiliary power supply is connected in series with a switch SW5 between the two other points of the bridge. A pair of switches SW1 and SW3 on the opposed arms of the bridge is simultaneously turned ON or OFF. Likewise, the other pair of switches SW2 and SW4 is simultaneously switched ON or OFF. Selectively turning ON one of the pairs of switches allows the polarity of current supplied from the auxiliary power supply to the gradient coil to be changed arbitrarily. The switch SW5 is turned ON during the interval when the current is rising or falling. The auxiliary power supply supports the main power supply so as to allow the supply current to the gradient coil to rise fast.
With the echo planar imaging, an alternated gradient magnetic field whose polarity alternates fast is needed. The waveform of a gradient magnetic field is shaped into a sinusoidal waveform etc. To change the current polarity in succession, the fast switching of the switches SW1 to SW4 is required.
However, the fast switching causes switching noise, which deteriorates the signal-to-noise ratio in magnetic resonance image data.